<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div.main {
      background: #161;
      height: 200px;
    }

    div.search {
      display: inline-block;
      /* vertical-align: middle; */
    }

    div.main::after {
      content: "";
      display: inline-block;
      /* background: yellow; */
      height: 100%;
      vertical-align: middle;
    }
  </style>
</head>

<body>
  <div class="main">
    <div class="search">
      <form action="">
        <input type="text" placeholder="找找看">
        <span class="button">search</span>
      </form>
    </div>
  </div>
</body>

</html>